The present invention relates to a device for accurately controlling the tension of a wire electrode in a wirecut electrical discharge machine.
FIG. 1 illustrates a wire electrode driving system for a conventional wirecut electrical discharge machine. As illustrated in FIG. 1, the conventional wirecut electrical discharge machine includes numerous parts, namely, a workpiece 1, a wire electrode 2 for machining the workpiece 1, a wire bobbin 3 where the wire electrode 2 is spooled, a torque motor 4 attached to the shaft of the wire bobbin 3, brake rollers 5a and 5b for applying tension on the wire electrode 2, a brake motor 6 directly connected with the brake roller 5a, a tachometer 7 for detecting the speed of the brake motor 6, an upper pulley 8 for supporting the wire electrode 2 above the workpiece 1 and changing the running direction thereof, a lower pulley 9 for supporting the wire electrode 2 under the workpiece 1 and changing the running direction thereof, wire electrode pinch roller 10a and take up roller 10b for driving the wire electrode 2, a constant-speed motor 11 directly connected with the wire electrode pinch roller 10a, an NC unit 12 for providing tension and running speed command values for the wire electrode 2, a comparator 13 for comparing the tachometer 7 output and the running speed command value provided by the NC unit 12, an amplifier 14 for matching the characteristic value of the tachometer 7 output and that of the running speed command value provided by the NC unit 12, and a compensator 15 for compensating for the tension command value provided by the NC unit 12 in accordance with the output of the comparator 13.
The operation of the conventional wirecut electrical discharge machine illustrated in FIG. 1 will now be described.
The wire electrode 2, which is spooled off the wire bobbin 3, is fed through a gap between the pair of brake rollers 5a and 5b where tension is applied. The running direction of the wire electrode 2 is then changed by the upper pulley 8 which is located above the workpiece 1. As a result, the wire electrode 2 is supplied to a machining area where the workpiece 1 can be machined. The running direction is again changed by the lower pulley 9 placed under the workpiece 1. Finally, the wire electrode 2 is wound around pinch roller 10a and take up roller 10b to pull the wire electrode 2 through the machine.
The wire bobbin 3 is driven by the torque motor 4 to apply tension of about 200 grams to the wire electrode 2 so
that the wire electrode 2 does not slack between the wire bobbin 3 and the brake rollers 5a and 5b. The brake roller 5a is furnished with the brake motor 6 to provide tension on the wire electrode 2 in the machine. The brake roller 5a is coupled to the brake motor 6 which controls the torgue with which the brake roller 5a moves in accordance with the tension command value provided by the NC unit 12.
The speed of the wire electrode 2 through the brake rollers 5a and 5b is detected as a voltage by the tachometer 7 installed on the brake motor 6. The voltage detected by the tachometer 7 includes an alternating-current (AC) component and a direct-current (DC) component. The output of the tachometer 7 is compared by the comparator 13 with the speed command value for the wire electrode 2 given by the NC unit 12, provided that the output of the tachometer 7 is compensated by the amplifier 14 in order to match the characteristic of the particular tachometer 7 output with that of the running speed command value provided by the NC unit 12. The output of the comparator 13 and the tension command value provided by the NC unit 12 are operated on by the compensator 15.
The pinch roller 10a is coupled to the constant-speed motor 11. The speed of the constant-speed motor 11 determines the running speed of the wire electrode 2 through the machine in accordance with the running speed command value provided by the NC unit 12.
The conventional wirecut electrical discharge machine is configured to keep the tension of the wire electrode 2 stable relative to dynamically varying tension (an AC component). The dynamically varying tension is, for example, a tension variation caused by the natural frequency of the wire system due to a disturbance, such as a machining counterforce.
When the tension variation (AC component) causes the wire recovery speed through the pinch roller 10a and take up roller 10b to be higher than the speed of the wire electrode 2 through the brake rollers 5a and 5b, the tension of the wire electrode 2 increases. Consequently, the comparator 13 outputs a positive signal, and the compensator 15 functions to reduce the tension. On the other hand, when the tension variation causes the recovery speed of the wire electrode 2 at the pinch roller 10a and take up roller 10b to be lower than the speed of the wire electrode 2 at the brake rollers 5a and 5b, the tension of the wire electrode system is decreased and the comparator 13 gives a negative signal so that the compensator 15 operates to raise the tension. Thus, the dynamically varying tension is controlled to be constant.
In the conventional wirecut electrical discharge machine configured as described above, the tension of the wire electrode 2 changes if a difference is produced between the running speed of the wire electrode 2 through the brake rollers 5a and 5b and the speed command value from the NC unit 12. However, the conventional machine cannot completely eliminate statically varying tension (the DC component) which occurs when the wire electrode stretches. For example, when tension is applied to the wire electrode 2, the wire electrode 2 stretches, causing the running speed of the wire electrode 2 at the brake rollers 5a and 5b to be lower than that at the pinch roller 10a and take up roller 10b. For this reason, the DC component of a voltage difference equivalent to the running speed difference is generated in the comparator 13. As a result, the compensator 15 undesirably operates to reduce the tension in accordance with the DC component.
In addition, if the generated voltage characteristic is not proportionate to the tachometer speed, a difference occurs between the voltage provided by the tachometer and the speed command value for the wire electrode, which changes the tension of the wire electrode. Further, if the tachometer output characteristic is not matched extremely accurately with the running speed command value, a DC component of the voltage difference is produced in the comparator 13 and changes the set tension of the wire electrode 2, leading to difficulty in the adjustment of the amplifier.
The running performance of the wire electrode 2 employed for machining the workpiece in the wirecut electrical discharge machine has great influence on machining accuracy, and in particular, the tension of the wire electrode 2 is an essential factor in determining the running performance. Further, to enhance the straightness of the wire electrode, the tension of the wire electrode is ordinarily set to a slightly higher value, to the extent that the wire electrode will not be broken. Therefore, if the tension of the wire electrode running system falls below the set value, problems will arise, e.g., the desired machining accuracy cannot be achieved. Conversely, if the tension of the wire electrode running system rises above the set value, the wire electrode will be broken or other problems may occur.